Salt content in food sold by six fast-food companies in Australia has dropped over a four-year period, a study has shown.

The study, published today in the Medical Journal of Australia, found the salt content of fast-food products dropped by 2 to 3 per cent between 2009 and 2012.

During that period researchers monitored the salt content of 300 products sold by six fast-food companies in Australia, including McDonald's, KFC and Pizza Hut.

Study co-author Dr Elizabeth Dunford, from the George Institute for Global Health, says the results of the study are encouraging.

"Although we found in our study salt levels had come down over a last four years, which is encouraging, there are some products that can give you your entire daily salt intake in one hamburger or sandwich," she told ABC News Breakfast.

"We are looking at a 2 to 3 per cent decrease every year, [so] overall, depending on the chain, it could be anywhere between a 5 and 15 per cent drop in salt levels."

Dr Dunford says the average Australian has a higher salt intake than what is recommended by the Federal Government.

"The Australian Government recommends an upper level of intake of 6 grams of salt per day - [the] suggested dietary target [is] 4 grams of salt per day," she said.

"The average Australian is eating around 9 grams of salt per day.

"If you are having that much salt per day, your blood pressure is higher than it should be and increasing over time.

"We know high blood pressure is a leading cause of cardiovascular disease in Australia."

Dr Dunford says while the decrease in salt levels is good news, an increase in portion sizes and the introduction of unhealthy side dishes is concerning.

"One of the fast food companies seemed to increase [salt] over the period and that was Pizza Hut," she said.

"That wasn't because of an increase in their standard pizza range, it was because of the introduction of very salty side dishes like chicken pieces that you could purchase alongside your pizza, and so that's what we were concerned about overall."

Dr Dunford says the Government should be encouraging fast-food chains to make bigger improvements.

"The fast-food industry, as we've shown in the study, has started to do things without government leadership, but 2 to 3 per cent is really not enough to make a significant difference at this stage," she said.

"So we think that with government engagement and stronger processes behind it, that more can be done so that consumers can get healthier products."

She said that could include changes to labelling.

"That would be wonderful," she said.

"We do think consumers find it very difficult right now to understand what's in the foods they're eating and we'd like to see the dialogue started between the Government and the industry."